Varsity Changing gear
 

There was a call in the climb somewhere 8 to 10,000 "Changing into S gear". Where upon power ran down for about 5 seconds and then returned to normal.
What was going on? Was the "driver" actually changing up a gear or in the car sense down a gear to spin the supercharger faster? Or engaging the superchargers which had not been required at the lower altitude or engaging a second stage of supercharging?
From the kid down the back who was too shy to ask then.

Indeed the pilot was shifting gears on the superchargers (I guess it would be down) to get them spinning faster and restore some of the power lost because of the thinning air.

I am not familiar with the Centauri on the Varsity but many American radials have two speed superchargers: High and low. Not sure what 'S' gear stands for here, but perhaps someone else can enlighten us.

A Varsity with Centauri might have been an interesting combination!!! - not too sure the wings would have stayed attached though! Hercs - 238s, if memory serves, would have been nearer the mark.

Fading memory here, but I think that 'S' gear was fully Supercharged, and 'M' gear was Medium supercharged.

Cornish Jack was close - Bristol Hercules 264s.

spekesoftly
Yes you have reawakened some cells. Not all Captains made the same call, some mentioned M gear.
Where & when was your association with Varsities?

Guess that's what happens when you go replying without doing any proper research first! I knew they were Bristols.....

I'll go and wash me mouth now...:p

To quote from the Varsity Pilot's Notes:

Oops pressed the wrong button! ' After take off, to commence climbing, reduce r.p.m. to 2400 and set the throttles at the R.B. gate (+9 lb/sq.in). As height is increased above about 3,000ft I.S.A., the indicated boost will fall. When the boost has fallen to +7 lb/sq.in (at approximately 5,250 feet I.S.A.), 'S' should be selected if the climb is to be continued. Before selecting 'S' the boost should be reduced to 0 lb/sq.in. After selecting 'S' gear, open the throttles to the R.B. gate, when the boost should be restored to +9 lb/sq.in. The full throttle height using +9 lb/sq.in and 2400 r.p.m in 'S' gear is approximately 11,000 feet I.S.A., after which it falls steadily to about +2 lb/sq.in at 20,000 feet I.S.A.' Usually the aircraft was operated in 'M' as we didn't often climb too high; if a high cruise was needed the above procedure was carried out.

Looking up a book by R-R on supercharging, and the Merlin XX in particular, MS gear ratio was 8.15:1 and used for t/o, SHP of 1318, BHP(at prop) 1132 at 50 insHg(+9lbs) at 3000 rpm.
Running the same boost and rpm in FS (9.49:1) would only give1236SHP, and 1020 BHP(prop), figures for sea level. Changing to FS gear when the boost began to fall would restore the power to peak at around 20000ft.
Although the higher gear ratio restores the power , this is due to improved intake efficiency, and reduced exhaust back-pressure
So , changing to FS gear is speeding up the supercharger from 24000 rpm to 28000 rpm, if you do it too early , more power is used from the SHP to drive it resulting in an overall loss, at low level. The reason is that it is actually like going into overdrive/ top gear too early, as the engine is running at a constant speed, and cannot change to accomodate the extra load.

Hope that explains it!!

For you Varsity/ Valetta guys, memory says after t/o , one reduced RPM first and then boost, against normal practice of boost ,then RPM??:ok:

Baron, thanks, your corrected post answered my question- just shows the grey-cells still have some life!!:ok: